1) Field of the Invention
The present invention relates to an optical sensor that receives the specular reflection light of incident light irradiated onto an illumination object, and an image formation apparatus that employs the optical sensor.
2) Description of the Related Art
An image formation apparatus such as that disclosed in Japanese Unexamined Patent Application Publication S64-35455, for example, is known as an image formation apparatus using an optical sensor. In this type of image formation apparatus, a density-detecting toner patch (reference pattern) is created on the surface of an image carrier such as a photosensitive body in order to obtain a stable image density, and the density of the patch is detected by an optical sensor. In this image formation apparatus, image density control is performed on the basis of the detection results of the optical sensor to adjust the development potential by modifying the fabrication light intensity for latent image formation, charging bias, developing bias, and so on, or to adjust the target toner density value inside the developing machine when a two-component developing system is used. The optical sensor uses the reference pattern as a detection object, and is therefore known as a P (pattern) sensor. A reflection optical sensor comprising light emitting unit and light receiving unit is typically used as the optical sensor.
A type of reflection optical sensor is known which detects specular reflection light generated when the light irradiated onto an illumination object is specularly reflected. This type of reflection optical sensor is disclosed in Patent Document 1 and so on. Taking a case in which toner density (the toner adhesion amount) on a photosensitive drum is detected, the detection principle employed when a reflection optical sensor which detects specular reflection light is used as a P sensor is as follows.
When no toner adheres to the surface of the photosensitive drum (illumination object), the incident light is specularly reflected by the surface of the photosensitive drum, and specular reflection light is received by a light-receiving element in accordance with the reflectance of the surface of the photosensitive drum. Conversely, when toner adheres to the surface of the photosensitive drum, the incident light is absorbed by the toner or reflected diffusely by the toner. Hence, in cases where incident light is blocked by toner before reaching the surface of the photosensitive drum, or specular reflection light from the surface of the photosensitive drum is blocked by toner before reaching the light-receiving element, no specular reflection light is received by the light-receiving element. Thus the amount of light received by the light-receiving element decreases as the amount of toner which adheres to the surface of the photosensitive drum increases. Accordingly, the toner adhesion amount on the surface of the photosensitive drum can be detected on the basis of the amount of light received by the light-receiving element.
An optical sensor using lead-type elements, for example, is also known as a conventional reflection optical sensor. In an optical sensor using lead-type elements, a light-emitting element and a light-receiving element are fixed in a resin case formed with an indentation which matches the elements. The light-emitting element and light-receiving element are connected to a substrate by lead wires, and thus light emitting signals or light receiving signals are exchanged with the main body of the apparatus. In an optical sensor using lead-type elements, the elements are fixed inside a resin case, and hence the positioning accuracy of the light-emitting element and light-receiving element is dependent on the manufacturing precision of the resin case. Since the resin case is a molded product, irregularities in the manufacturing precision are virtually nonexistent. Hence by creating a resin case which matches the optical system, a reflection optical sensor with a high degree of element positioning accuracy can be obtained.
An optical sensor using surface mounted elements, in which the elements are placed directly onto a substrate without the use of leads, is also known. By using these surface mounted elements, reductions in cost and improvements in productivity can be achieved in comparison with conventional lead-type elements, and moreover, the entire optical sensor can be reduced in size.
However, the following problems are encountered in conventional optical sensors and image formation apparatuses such as those described above.
FIG. 23A and FIG. 23B are schematics for illustrating states that toner is transferred to the surface of a photosensitive drum 5 serving as an illumination object. When the surface of the photosensitive drum 5 is irradiated with incident light L1 from a light-emitting element, not shown in the drawing, of a P sensor, and the incident light L1 is not obstructed by toner T, the incident light L1 is specularly reflected by the photosensitive drum surface, and the resultant specular reflection light L2 is received by a light-receiving element not shown in the drawing. However, the incident light in the region of the drawing indicated by diagonal lines is obstructed by the toner T and cannot reach the light-receiving element. When detecting specular reflection light, the optical path of the incident light must be inclined in relation to the normal direction Z of the photosensitive drum surface. Hence a surface area S1 of the part of the photosensitive drum surface which does not contribute to the specular reflection light that is received by the light-receiving element is larger than the surface area of orthogonal projection of the toner T in relation to the photosensitive drum surface, or in other words a surface area S0 of the part of the photosensitive drum surface that is actually occupied by toner. More specifically, the proportion of the surface area S1 of the part of the photosensitive drum surface which does not contribute to the specular reflection light that is received by the light-receiving element in relation to the surface area S0 of the part of the photosensitive drum surface that is actually occupied by toner (hereinafter, “the shadow factor”) increases. Hence, as shown in FIG. 23B, when a toner particle T2 approaches another toner particle T1, the specular reflection light of a space S2 between the toner particles is not received by the light-receiving element even though no toner exists in the space S2. As a result, at the stage when toner is transferred to the photosensitive drum at intervals of approximately the space S2, it becomes difficult to detect the adherence of more toner using a P sensor. Hence in regions where a large amount of toner is transferred to the photosensitive drum surface, the sensitivity of the P sensor which detects specular reflection light decreases, and thus it becomes difficult to detect the toner adhesion amount.
FIG. 24 is a graph for illustrating a relationship between an amount of black toner transferred to the surface of the photosensitive drum and the output voltage of the P sensor that detects specular reflection light. FIG. 25 is a graph for illustrating a relationship between an amount of color toner transferred to the surface of the photosensitive drum and the output voltage of the P sensor that detects specular reflection light. As can be seen from these graphs, in the case of both black toner and color toner, variation in the output voltage of the P sensor in relation to increases in the amount of transferred toner is sufficiently large up to an adhesion amount of approximately 0.3 mg/cm2, and hence the toner adhesion amount can be detected. However, when the adhesion amount exceeds this level, there is substantially no change in the output voltage of the P sensor, and hence the toner adhesion amount cannot be detected. Note that in the case of color toner, as shown in FIG. 25, the output voltage of the P sensor switches from monotonous decreasing to monotonous increasing on reaching approximately 0.4 mg/cm2. This is a phenomenon which occurs due to a property according to which black toner absorbs light, but color toner diffusely reflects light. In other words, according to this phenomenon, in the case of color toner, diffuse reflection light that is diffusely reflected by the color toner is received by the light-receiving element as well as specular reflection light, and the amount of diffuse reflection light received by the light-receiving element increases as the toner adhesion amount increases.
Particularly in recent years, the miniaturization of toner particle diameter has progressed and the roundness of toner has improved, and hence limits on the detection of toner adhesion amounts by a P sensor which detects specular reflection light have become substantially narrower. To explain this using the graphs shown in FIG. 24 and FIG. 25, the point at which variation in the output voltage of the P sensor in relation to the toner adhesion amount substantially ceases is currently shifting to a lower toner adhesion amount.
More specifically, in the case of a fine particle toner having a weight average particle diameter of 8 micrometers or less, the surface area of a recording paper surface that is covered when a single toner particle is spread over the recording paper by the heat and pressure generated upon adhesion is small. Accordingly, in order to obtain a uniform image density, more toner must be used as the particle diameter of the toner decreases. Hence, when detecting the toner adhesion amount on the surface of an image carrier so as to obtain a desired image density, regions in which the toner adhesion amount is large in accordance with the small particle diameter of the toner must be detected with good sensitivity. In other words, since the toner adhesion amount detection range of the P sensor shifts toward the high adhesion amount side as the particle diameter of the toner decreases, limits on the detection of the toner adhesion amount by the P sensor become substantially narrower. Moreover, as shown by the graphs in FIG. 24 and FIG. 25, the toner adhesion amount is typically expressed as a toner weight per surface area unit, and the toner weight is proportionate to the toner volume. In this case, if the toner radius is assumed to be R, then the toner volume decreases in proportion to 1/R3 as the toner diameter is reduced, and hence the toner weight also decreases in proportion to 1/R3. As a result, the point at which variation in the output voltage of the P sensor in relation to the toner adhesion amount substantially ceases shifts to a lower toner adhesion amount, and hence limits on the detection of the toner adhesion amount by the P sensor become even narrower.
In the case of toner having a high degree of roundness, in which the average roundness is 0.93 or more, the aforementioned shadow factor tends to increase. As a result, the sensitivity of the P sensor which detects specular reflection light in relation to regions on the photosensitive drum surface having a large toner adhesion amount decreases, making it difficult to detect the adhesion amount in such regions.
An optical sensor used to detect an amount of toner transferred to a photosensitive drum was described above, but any optical sensor which receives specular reflection light to detect the amount of an illumination object which does not specularly reflect light existing on an object which specularly reflects light also possess similar problems regarding the narrowness of the detectable range.
Further, the positioning accuracy of a surface mounted light-emitting element or light-receiving element is dependent on the positioning accuracy when the surface mounted element is surface mounted on a substrate. However, this positioning accuracy is limited, and a degree of positioning accuracy as high as that obtained with a lead-type element, where element positioning is performed using a resin case, cannot be achieved. Hence in a conventional optical sensor using surface mounted elements, irregularities occur in the positioning accuracy of the light-emitting element or light-receiving element even in identical products. As a result, a uniform light receiving sensitivity cannot be obtained when an illumination object is irradiated with light from the light-emitting element. In other words, irregularities occur in the detection characteristic of a sensor even when the product is identical.